Solid, natural wood is a relatively expensive material, and thus items made from natural wood are generally more expensive than items made from alternative materials such as plastic or wood composite. In addition, solid wood provides aesthetic qualities that are desirable to many consumers. As the price of natural wood has increased, the market for manufactured products that simulate natural wood has grown. For example, door skins, wainscot, molding, trim, and the like are often made from composite materials, such as fiberboard, rather than from solid wood.
Hollow core doors simulating natural, solid doors are well known in the art. Such doors are often formed from two thin sheets of fiberboard, referred to as “door skins”, which are secured to opposite sides of a peripheral frame. The resulting door has the thickness of a solid wood door, but is hollow in the middle or has a solid core. The hollow space may be filled with corrugated pads, a contoured wood fiber core, insulation or another material if desired. The door skins may have a smooth, planar surface (i.e. flush door skins), a textured surface, or a contoured surface (i.e. molded door skins). Molded door skins are often formed to have portions simulating stiles, rails and panels, as found in traditional wooden rail and stile doors.
Wood composite articles, such as door skins, are somewhat similar to natural wood in strength and density, but lack the appearance of natural wood, especially the color, grain and/or inlay patterns that are considered desirable by many consumers. Therefore, such molded articles are often painted to enhance the appearance of the composite material used to form them. If a natural appearance is desired, a wood veneer may be bonded to the surface of the article. For example, boards used to make “flat-pack” furniture often comprise a core of chipboard, with a wood veneer secured to the exterior surface of the core to give the appearance of a solid, natural wood board. Such a board is often lighter and less expensive than a solid board of comparable dimensions, which may be advantageous depending on the application of the board.
The veneer may comprise a thin sheet, or plies, of solid wood. Alternatively, the veneer may be a plastic-based material on which an image of wood is applied. The veneer is bonded to the underlying core substrate either before or after the article is manufactured. Veneers are widely used as coatings to create simulated wood for tables, doors, and other furniture articles. Papers and foils may also be used to simulate the appearance of wood grain. However, the application of veneers, papers and foils is often time consuming, and, especially in the case of papers and foils, can produce an unacceptable product if great care is not taken in the application of the materials. This increases the manufacturing cost of such articles and results in varied aesthetics.
Another method of simulating a wood grain pattern provides for printing the wood grain pattern on the surface of a flat article using a patterned roller, known as offset-gravure printing, that transfers paint onto the article's surface. Alternatively, cylinders engraved with a desired wood grain pattern may be used. However, such printing methods are generally complex, and require the use of a different set of rollers or cylinders for each desired pattern or for differently shaped articles being printed. The rollers or cylinders produce doors having identical patterns with small repeats due to cylinder size. In addition, the engraved cylinders and rollers are relatively expensive, but not overly reliable to hold close register.
In an attempt to provide more varied patterns without the use of multiple rollers or cylinders, some methods provide for the use of jets of fluid to create random wood-grain-like patterns on flat panels of various materials. For example, one such method is disclosed in U.S. Pat. No. 4,849,768. Other methods including printing on flat fiberboard using an ink jet printhead, such as disclosed in U.S. Pat. No. 5,683,753 and U.S. Pat. No. 6,095,628.
However, prior art ink jet printing methods have failed to achieve satisfactory image quality on a printed article, particularly when printing on fiberboard. While it may sometimes be possible to produce low-resolution simulated wood grain on planar surfaces, such as flush door skins, it has heretofore not been possible to produce high-quality images directly on contoured surfaces. Instead, when high quality images are needed, it is necessary to print such images on paper or film and then attach the paper or film to the surface of the substrate in a labor-intensive lamination step. Thus, for example, doors having high quality images are generally made in limited quantities, when the cost can be justified. Moreover, on contoured surfaces, such as molded door skins, it has not heretofore been possible to produce either a realistic wood grain or other images in the recessed and/or raised contoured portions of the skin.
It is known from prior art patents such as U.S. Pat. No. 6,360,656 to Kubo that a surface having a raised feature can be ink jet printed if the ink application rate is varied as a print head passes over the feature. However, this method requires that the distance between the printhead and the raised feature be carefully controlled, and therefore sensors are required to accurately measure the distance between a printhead and the surface being printed. If the feature is a recessed portion, such as a molded channel, additional problems arise using the method disclosed by Kubo. First, the width of the channel may be less than the width of the printhead, making it impossible to lower the printhead into the channel to maintain the required spacing between the printhead and the surface being printed. Second, turbulence surrounding ejected droplets of ink may be magnified by the narrow channel, making it difficult to control the placement of ink droplets.
Increasing the distance between the printhead and recessed portions of a surface to be printed, to overcome problems associated with Kubo, have also failed to achieve a adequate quality image. One of the problems of increasing the distance of travel of the ink droplets in the region of a recess is that after a short distance of travel from the printhead nozzles, there is breaking of the droplets due to the viscosity of the air and the relatively small size of the droplets. As the droplets lose momentum, they become increasingly susceptible to air currents that move the droplets away from their intended path. This ultimately leads to errors in droplet placement and thus reduction in image quality. Furthermore, even if the distance between the printhead and surface to be printed is relatively short, a first droplet that is emitted from a nozzle sometimes interacts with the subsequent droplet emitted from the same and/or adjacent nozzle because the subsequent droplet moves in the slipstream of the first droplet and thus speeds up relative to the first droplet. These effects are magnified in confined areas such as within a recessed portion. This affects droplet placement and image quality.
If a curtain of ink droplets is deposited, as for a multi nozzle printhead, the droplets often slow down because their momentum is transferred to the air. This effect can act as an “air pump,” causing the droplets at the edge of the curtain to be pulled in towards the other droplets, causing turbulence and droplet interaction. Droplet placement and image quality may be adversely affected. Furthermore, if the article to be printed is moving relative to the printhead, there may be additional detrimental effects on droplet placement. All of these effects combine to reduce print quality.
It is therefore desirable to provide a method of printing either wood grain images or other graphic images on the surface of a flush or molded article, such as a door skin, in a manner that produces high quality images over the entire exterior surface being printed.